a) Field of the Invention
The present invention relates to a method of treating wooden poles, particularly utility poles, to improve the service life thereof, and further relates to poles treated in this manner.
b) Description of the Prior Art
The treatment of wood products with preservatives is a necessary and common practice for the appropriate use of wood under service conditions that promote fungal and insect attack, e.g. in utility poles that have their butt ends buried in the ground. The most susceptible zone for decay in such a wooden pole is within two feet above and below the ground level, because in this zone the moisture content is frequently above 30% (which corresponds to the fiber saturation point of the wood) and sufficient oxygen is available.
In the prior art, wooden poles are typically air dried to an appropriate moisture content before pressure treatment with preservatives. The sapwood must be dried to a moisture content below the fibre saturation point to ensure adequate penetration and retention of the oil-borne preservative. However this does not necessarily preclude subsequent decay in service. The active ingredients of the preservative can break down and be released from the pole, and the preservatives lose their efficacy over time, and particularly after long service in the field. Such release of preservative to the ground could have a negative impact on the environment, so that in some areas the use of such treated wooden poles is not permitted.
Additionally it is well known that wood shrinks as it becomes drier, so that in service as the wooden pole dries checks develop, and these checks are often deep enough to provide passage for wood destroying fungi or insects to penetrate into the core region of the pole, beyond the zone that has been impregnated with the preservative. Damage as a result of storms or impacts such as traffic accidents can also occur.
Eventually all wood poles have to be replaced or repaired after a period of years which varies according to some of the conditions outlined above. Typically, the life expectancy of a utility pole is within the range 30 to 40 years. Since the number of wooden utility poles in use in Canada and the U.S.A. is conservatively estimated to,be over one hundred million, the replacement rate for such poles must amount to several million per year.
It will be appreciated that to replace a utility pole and transfer facilities from an old pole to-the replacement pole is very costly, especially for a pole that supports power transmission lines. Additionally, where a new transformer is required in a given area, it is clearly less expensive to install it on an existing pole than to erect a new pole, but this cannot be done if the existing pole has deteriorated to the extent that it is not strong enough to support the transformer.
In view of the large number of poles in use, any measure to upgrade and extend the service life of such poles will result in large savings in labour and material costs.